The present invention relates to a tandem master cylinder with central valves for hydraulic brake systems of automotive vehicles.
Such a master cylinder is known from U.S. Pat. No. 4,939,901. The master cylinder described therein contains a central valve each both in the primary and secondary master cylinder piston. In the inactive position, i.e. when the vehicle's brake pedal is not depressed, the central valves, each comprising a valve seat, a valve member, a cylindrical valve spring biasing the valve member, and a valve stem connected to the valve member, are held open by cylinder pins abutting the valve stems. When the tandem cylinder is actuated, the pistons move so that the valve seats are moved away from the pins and the central valves close in order to enable a pressure-build-up in the pressure chambers of the master cylinder.
The concept of central valves with valve stems is more durable under high pressure than other types of master cylinders. However, the design requires an increase in length of the master cylinder housing due to the elements that need to be accommodated, for instance the valve stems and cylindrical valve springs. However the packaging space under the hood of a car is limited, and a smaller build of any component would be a great advantage. Additionally, the flow of brake fluid through open central valves is rather small and determined by the cross-section of the valve opening and the shape of the passage in connection with the mushroom-shaped valve member, which causes turbulences in the fluid.
It is an object of the present invention to provide a tandem master cylinder which allows for a shorter build and which is more cost-effective to manufacture. Additionally, it is desirable to optimize the flow through the vale opening.